Power supplies are used in a wide variety of applications. Many power supplies, including for example those used in residential electricity meters, deliver power levels in the range of 1-2 watts. These power supplies typically use linear transformers at 50-60 Hertz (Hz) as the main source of voltage conversion. The use of a linear transformer may reduce overall cost and eliminate the need for peripheral equipment normally associated with protection from voltage transients on the input voltage.
In a conventional full-wave rectifier power supply, filter capacitors typically sit directly on the output of a diode bridge. The filter capacitors draw current from the transformer at any time the output voltage of the transformer is higher (by two diode drops) than the direct current (DC) voltage on the capacitors. The energy delivered to the capacitors is proportional to the difference in the transformer output voltage and the rectified DC voltage and inversely proportional to the impedance of the transformer. At all times in the alternating current (AC) voltage cycle where the transformer output voltage is less than the rectified DC voltage, there is no energy delivered to the capacitors.
Peak power requirements in many applications, such as for example new utility metering applications, may reach six watts for sustained periods of time. The majority of power supplies that can source such peak power levels are designed as off-line, switching power supplies. A typical off line switching supply provides some filtering of the 120 or 240 volt alternating current (VAC) input voltage and rectifies the AC voltage to create a high voltage DC. The high voltage DC feeds a converter chip and high frequency transformer to develop a switching conversion from the high voltage DC down to low voltage for use by the device in which the power supply is used.
One problem associated with off-line, switching power supplies is protection of the electronics from high voltage transients. Because the supply is being required to deliver significant power levels, the allowable input impedances must be relatively low. The result of these low input impedances is higher fault currents associated with voltage transients and correspondingly higher stresses on all front-end components. Parts costs and reduced reliability reflect the higher stresses.